Re: Finding the Bottleneck
I in no way pretend to know a lot about the kernel and the specific ways
it handles free memory and caches, but i just look at it from a "logical"
point of view.
Hopefully I'm not too far off course in the assumptions i make! Hope Rik
can clarify this not just for me but for everyone thats been following
this thread. Since he is the expert on this, he's the authority!
----- Original Message -----
From: "Russell Coker" <firstname.lastname@example.org>
To: "Jason Lim" <email@example.com>
Cc: <firstname.lastname@example.org>; <email@example.com>
Sent: Monday, June 11, 2001 6:43 PM
Subject: Re: Finding the Bottleneck
On Saturday 09 June 2001 20:04, Jason Lim wrote:
> I'm not exactly sure how the Linux kernel would handle this.
> Right now, the swap is untouched. If the server needed more ram,
> wouldn't it be swapping something... anything? I mean, it currently has
> 0kb in swap, and still has free memory.
That is a really good point. What you say makes a lot of sense. However
the Linux kernel policies on when to free cache and when to swap are
always being tweaked and are very complex.
I have CC'd this message to Rik. Rik wrote most of the code in question
and is the expert in this area.
Rik, as a general rule if a machine has 0 swap in use then can it be
assumed that the gain from adding more RAM will be minimal or
non-existant? Or is my previous assumption correct in that it could
still be able to productively use more RAM for cache?
As a more specific issue, assuming that there is memory which is not
being touched (there always is some) will that memory ever be paged out
to allow for more caching?
I believe that Jason is using kernel 2.2.19, but I would like to have
this issue clarified for both 2.2.19 and 2.4.5 kernels if possible.
> Anyway... as for the raid solution, is there anything I should look out
> for BEFORE i start implementing it? Like any particular disk or ext2
> settings that would benefit the mail queue in any way? Don't want to
> get everything set up, only to find I missed something critical that
> you already thought of!
There is an option to mke2fs to tune it for RAID-0 or RAID-5. I'm not
sure if it provides much benefit though, and it does not help RAID-1
(which is the RAID level you are most likely to use).
I suggest that you firstly run zcav from my bonnie++ suite on your new
hard drives. Then allocate partitions for the most speed critical data
in the fastest parts of the drives. Then use RAID-1 on those partitions.
http://www.coker.com.au/bonnie++/ Bonnie++ hard drive benchmark
http://www.coker.com.au/postal/ Postal SMTP/POP benchmark
http://www.coker.com.au/projects.html Projects I am working on
http://www.coker.com.au/~russell/ My home page